Small-molecule inhibitors of the ERK signaling pathway: Towards novel anticancer therapeutics

The Ras→Raf→MEK(mitogen-activated kinase kinase)→ERK (extracellular-signal-regulated kinase) signalling pathway is one of at least five mitogen-activated protein kinase (MAPK) pathways that control several fundamental cellular processes, driving proliferation, differentiation and cell survival.[1–3] Signal transduction through this particular pathway, which is depicted in Figure 1, is initiated by the binding of a wide variety of ligands, including hormones and growth factors, to receptor tyrosine kinases (RTKs). This leads to the activation of Ras proteins (H-, K- and N-Ras isoforms) previously anchored in the plasma membrane by earlier post-translational reactions, e.g. farnesylation. Subsequently, the Ras proteins are induced to exchange their bound GDP for GTP, which leads to a conformational change in Ras and the initiation of a three-stage phosphorylation cascade that climaxes with the activation of ERK1/2. First, the Raf family of kinases (A-, B- and Raf-1 isoforms), the best studied of which is Raf-1, is recruited to the plasma membrane. Upon its subsequent phosphorylation, Raf-1 then activates (phosphorylates) MAP / ERK kinase 1 and 2 (MEK1/2), which, in turn, activate (phosphorylate) ERK1 and ERK2 (p44 MAPK and p42 MAPK, respectively). ERK1/2 are activated through phosphorylation of both a threonine and a tyrosine residue, namely Thr202 and Tyr204 of ERK1 and Thr183 and Tyr185 of ERK2. MEK1/2 are the only known activators of ERK1/2 and are, thus, dual specificity kinases. Activated ERK1/2 then phosphorylate serine/threonine residues of more than 50 downstream cytosolic and nuclear substrates, leading to alterations in gene expression profiles and an increase in proliferation, differentiation and cell survival.[1–3] Figure 1 Schematic representation of the Ras→Raf→MEK1/2→ERK1/2 signalling pathway. GF = growth factor, RTK = receptor tyrosine kinase, Grb2 = growth factor receptor-bound protein 2; Sos = son of sevenless; P indicates a phosphorylated serine, ... There is now considerable evidence that links the dysregulation of the Ras→Raf→MEK→ERK pathway to the oncogenesis of human cancers. Ras is hyperactivated in around 30% of human cancers, most commonly the K-Ras isoform.[4] More specifically, Ras activating mutations have been reported in about 90% of pancreatic carcinomas, 50% of colon carcinomas, 30% of lung cancers and in around 30% of myeloid leukaemia cases.[4] Activating mutations of Raf have also been reported in around 7% of human cancers.[5,6] In particular, mutations of B-Raf have been observed in over 60% of melanomas, around 30% of ovarian cancer and in approximately 20% of colorectal carcinomas, as well as in several other malignancies at lower frequencies.[5,6] Constitutively activate MEK1/2 and ERK1/2 proteins are present in a relatively high number of human tumours, particularly those from the colon, lung, pancreas, ovary and kidney.[7] Since mutations of the MEK1/2 and ERK1/2 genes have not been observed in human tumours, it seems probable that the hyperactivity of these proteins is a consequence of their constitutive phosphorylation due to hyperactivation of upstream effectors, including receptors, Ras and B-Raf. In summary, the Ras→Raf→MEK1/2→ERK1/2 pathway is an appealing target for the development of potential anti-cancer therapeutics. Moreover, the pathway offers several junctures for signal transduction blockade; due to the converging functions of MEK1/2 and ERK1/2, specific inhibition of these proteins is particularly desirable. In this mini-review, some of the more prominent small molecule inhibitors of the ERK pathway will be presented, with a particular emphasis on those discovered within the last ten to fifteen years. In the first section, we shall discuss those inhibitors that target proteins upstream of ERK1/2, specifically Raf and MEK1/2. We will then shift to the main focus of this review, which is the direct inhibition of ERK1/2 through targeting either the ATP-binding site (ATP-competitive inhibitors) or the surface of ERK and blocking its protein–protein interactions with its substrates (non-ATP-competitive inhibitors).